Fill in the blank with the correct form of verb: You had better ............…

Concept: The semi-modal expression "had better" (used to give strong advice) is always followed by the bare infinitive - the base form of the verb, with no "to", no "-ing", and no past-tense or past-participle ending.

Application: In "You had better ............ your hands before you eat," the subject "You" is followed directly by "had better", so the blank must take the bare infinitive of the verb "wash" - the dictionary form with no ending added.

• "been washing" combines the past participle "been" with the "-ing" participle - this is the non-finite remainder of a present-perfect-continuous verb group (as in "has/have been washing") and needs a finite auxiliary such as "has"/"have" that the blank does not supply; a different verb-group shape from the bare infinitive that "had better" requires.

• "washing" is the "-ing" present-participle/gerund form, not the uninflected base form.

• "washed" is the "-ed" past-tense/past-participle form, not the uninflected base form.

• "wash" carries no ending at all - exactly the bare-infinitive shape that "had better" calls for.

Cross-check: Substituting another bare-infinitive-taking modal confirms the pattern: "You must wash your hands" and "You should wash your hands" are both grammatical with the base form "wash", showing that "had better" behaves the same way.